With the customary welding processes, in particular beam welding (electron beam welding, laser beam welding), there are two methods known:
a) Metal sheets are positioned precisely with respect to a travelling beam welding tool, clamped, and welded. This process is not a continuous welding process. PA1 b) Metal sheets, arriving from joggled planes to mechanical, high-precision templates, are introduced into and welded by a stationary beam welding tool. With this process the metal sheets are welded continuously. PA1 a) Relative to the direction of transport the slave sheet reduces its speed as a function of its retaining force, and this happens until the slave sheet is carried along by a fixed stop. Since the driven movement of the slave side is synchronized with the driven movement of the master side, the master and slave sheets are now positioned to one another in the direction of welding and continue to move towards the welding machine. PA1 b) The effects of the lateral forces on the slave sheet drive it against the master sheet, and hence abuts the master sheet.
Both methods entail considerable work in creating the requisite mechanical precision (approx. 0.03 mm) and pose problems with respect to mechanical wear incurred during the production. This wear leads to losses in quality when metal sheets are welded.
Previously known from DE 38 30 892 is a process for determining the relative position of a weld seam to a specifiable target position in order to be able to rectify the relative position of the welding laser beam to the weld seam. In the rectification process a collimated laser measuring beam around the seam path is alternately deflected and focused and, after reflection from the workpiece arrangement forming the weld seam, received by an optoelectronic sensor and transmitted to a repositioning device. This process serves to determine the position of a weld seam relative to a specifiable target position in order to be able to rectify the relative position of the welding laser beam to a weld seam. Here the deflecting frequency of the deflected laser measuring beam should be at least 100 Hz, whereby the intensity of the laser measuring beam is modulated with at least 200 kHz in such a manner that the laser measuring beam impinges on the workpiece arrangement at a distance of no greater than 10 mm from the axis of the welding laser beam, whereby a single light-sensitive element only is used as an optoelectronic sensor. This has as a result that feed and weld speeds of 100 mm/s can be reached, whereby, at this feed speed, the seam path can be determined in a specified grid. Modulation of the laser diode radiation and further, subsequent demodulation of the sensor signal, amplitude modulated in addition by the workpieces, facilitate the elimination of disruptive influences exerted by the laser welding plasma and the Planck's radiation from the weld pool.
In DE 38 30 892 a level of the technology is also described with which, in the cases of a line weld, the starting and end points of the weld seam are determined with the assistance of a measuring beam superimposed precisely along the axis of the main laser or welding beam. The points thus determined are entered into the control program for the main laser beam, and the seam is welded between the determined points in accordance with the program specified--in the assumed case, a line weld.
Described in addition in DE 28 30 892 is a weld tracking system for automated arc welding with which the prepared weld seam is scanned with an alternately focused laser measuring beam at a measuring frequency of no greater than 10 Hz across the weld seam, and the position of the laser welding beam or the electrode holder relative to the weld seam is rectified accordingly in the event of a deviation of the weld seam from the target position. Used as the sensor with this known weld tracking system is a photodiode cell. Owing to the large number of measuring points and the time required for the evaluation, this weld tracking system has a measuring accuracy of only 0.2 to 0.5 mm.
Further described is that the diameter of the focused spot of a focused operating laser or welding beam may at times only be 0.1 to 0.2 mm. However, owing to this very low expansion of the focused spot, the weld seams intended for laser welding likewise exhibit very small cross sections. Assumed, therefore, for the welding of such weld seams can be a measuring accuracy of half the diameter of the focused spot, i.e. of about 0.05 mm.
Following this assumption, therefore, the measuring precision achievable with the known weld tracking systems used for arc welding no longer proves sufficient. The feed speed achieved for laser welding can be from 100 mm/s to 6 m/min and, in general, can be up to ten times greater than the speed for arc welding. For this reason also, the measuring frequency of the measuring systems known for arc welding proved insufficient since the measuring signals yielded by the oscillating measuring beam do not occur sufficiently fast enough in order to be able to reposition exactly the main laser beam at the given feed speeds.
Previously known from DE 37 23 61 1 is a device for the continuous buttwelding of strips and sheets by means of at least one stationary laser beam with tension rollers arranged in pairs vertically to the direction of travel and on both sides of the strips or sheets to be welded, whereby these tension rollers have hollow axles whereon roller tubes arranged at fixed axial intervals are mounted on bearings, and every welding head of the laser beam welding device is equipped with at least one tension roller in the interior of the hollow axle and where the gap between the roller tubes and an aperture in the hollow axle serves as a passage for the laser beam. With this device, the focused spot of the laser beam is to be directed onto the sheet edges for welding in the area clamped by the tension rollers. Clamping the sheets or strips by means of large and stable rollers helps prevent the sheet edges from warping and the sheets from twisting in the weld area.
DE 38 01 626 concerns a rotating circular scanner, functioning on the principle of triangulation, as an optical seam position sensor for a burner with a primary beam. This primary beam, fed in particular from a laser light source, is emitted eccentrically to an axis of rotation, describes a mathematical cylindrical or conical mantle, and is directed onto the surface of the workpiece where it forms a light spot travelling in a circle on the workpiece and where it is detected with an optical observation device which is arranged within the cylindrical or conical mantle, pointed towards the site of the light spot, and inclined to the optical axis of the primary beam. This optical observation device forms an image of the light spot on a photodiode array, lateral effect diode, or such like (secondary beam), whereby for every circumferential position of the circular scanner the corresponding related value pairs, comprising the circumferential position and the secondary beam position on the photodiode array, the lateral effect diode, or such like, are determined and transmitted as a sensor signal to the evaluation electronics. The photodiode array, the lateral effect diode, or such like is designed for two dimensional operations and fixed in a stationary position, i.e. nonrotating, and serves to determine simultaneously both values of the named value pair. The previous publication also mentions the concentric arrangement of a seam position sensor. Moreover, circular scanners are described which, when being used as seam position sensors for burners, can be arranged eccentrically to the burner axis ahead of the direction of welding or concentrically with the burner. These previously known circular scanners could assist in gaining information for the weld seam position in relation to the burner, the direction of the weld seam, and the weld seam configuration. This information on the weld seam configuration may permit, for example, that the welding parameters adjust themselves independently with respect to the changed seam conditions or that an alarm signal is triggered for the attention of maintenance personnel.
DE 40 22 062 concerns a device for the controlled feed of strips and butt welding along their longitudinal edges, whereby for each strip there is at least one deflector roller with controlled travel along the axis. The last deflector rollers (in the direction of the strip before the welding point) for each strip are vertically displaced so far from each other in their parallel axes that the longitudinal edges of the strips fed over the deflector rollers lie with respect to each other in the position required for welding and on the same vertical plane and are laterally displaced so far that between their opposite end surfaces there is a gap between the longitudinal sides. Also one of the last reflector rollers is mounted on movable bearings at the height position and another on movable bearings along the axis, whereby a welding device is provided whose welding beam is directed on the weld position between the last deflector rollers. Provided in addition are control means serving to change the axial positions of the deflector rollers. Furthermore, there are scanning means present. Owing to the horizontally joggled arrangement of the last deflector rollers of which one can be displaced with respect to the other in the axial direction, precision regulation of the position of the strips to each other should constantly ensure a gap of narrow tolerance between the longitudinal edges during production. The axial position of the last deflector rollers, lying along the axis at a distance from each other, is constantly regulated via the mentioned scanning means on the longitudinal edges of the strips so that the gap width required is constantly kept. The distance between the longitudinal edges of the strip is determined via a sensor and the results of the measurement evaluated by a computer. These yield the specifications to the positioning units of the regulator rollers. Before the strips are abutted at the vertex of the two last deflector rollers there takes place a precision regulation. These processes are achieved via sensors which determine the positions of the longitudinal edges of the strips relative to each other. The measurement values thus gained via the sensors are likewise evaluated by the computer. These evaluated results then yield the specifications for the precision regulation for the positioning unit of the last deflector roller, which is repositioned in the axial direction when a deviation from the target position of the longitudinal edges of the strip is established. The intended result thereby is that a constant predefinable gap is always available between the longitudinal edges of the strip before commencement of the welding process.
EP-A-0 450 349 concerns a process for the continuous welding of abutted strips or sheets without fillers by means of a laser beam, whereby the strips in the area of the weld seam in the direction of the strip feed are cooled directly behind the weld focus, whereby the cooling intensity as a function of the width of the gap formed directly before the weld focus by the abutting edges of the strips in the direction of the strip feed is regulated in such a manner that the width of the gap remains within the specified tolerance values. Cooling both sides of the strips is also proposed. Also proposed in this preliminary publication is a device for the continuous welding of abutted strips or plates at their abutting edges by means of a stationary laser beam with tension rollers arranged in pairs on both sides of the strips for welding and vertically to their feed direction--these tension rollers form in the area of the abutting edges of the strips a gap through which the stationary laser beam is directed onto the abutting edges for welding--and with a measuring device--arranged in the feed direction of the strip directly before the weld focus--for the width giving the actual value of the gap formed by the abutting edges of the strips. Furthermore, a regulating device is provided which includes the named measuring device and a cooling device arranged in the feed direction of the strip behind the weld focus and acting on the strips in the area of the weld seam--whose cooling intensity as a function of the actual value supplied by the measuring device is set for the gap width in the sense of a constant gap width lying within specified tolerance values.